Heat exchanger using thermal convection tubes

ABSTRACT

A heat exchanger is constructed with the hotter fluid in a lower region and the cooler fluid in an upper region. A single wall separates the two regions. Vertically mounted thermal convection tubes extending from the hotter fluid to the cooler fluid through the wall act to transfer heat between the fluids. The thermal convection tubes contain a working fluid and provide for substantially isothermal heat transfer, the latent heat of vaporization of the working fluid being the primary mechanism responsible for this heat transfer.

United States Patent [72] Inventors David M. France 2,581,347 1/1952Backstrom 165/105 X Lombard; 308,197 11/1884 Rober 165/105 X MichaelPetrick,Jo1iet, both of III. 2,883,591 4/1959 Camp 165/105 X [21] Appl.No. 36,036 3,446,188 5/1969 Nozawa et a1. 122/32 Filed y 1 1 FOREIGNPATENTS [451 Patented 1 1 t a 't 165/105 [73] Assignee The United Statesof America as 500l33 939 Grea n represented by the United States AtomicPrimary Examiner-Albert Davls, J

Energy Commission Attorney-Roland A. Anderson [54] HEAT EXCHANGER USINGTHERMAL CONVECTION TUBES 1 Claim, 6 Drawing Figs.

[52] US. Cl 165/105, ABSTRACT; A heat exchange.- is constructed with thehone, 122/32 fluid in a lower region and the cooler fluid in an upperregion. [51] lnt.Cl F28d 15/00 A sing: l Se mates the two re i0nsvertica mounted P g y [50] Fleld of Search 165/105, thermal convectiontubes extending f the fl id to 122/367 A, 32, 33 the cooler fluidthrough the wall act to transfer heat between the fluids. The thermalconvection tubes contain a working [56] References C'ted fluid andprovide for substantially isothermal heat transfer, the UNITED STATESPATENTS latent heat of vaporization of the working fluid being the pri-2,893,706 7/1959 Smith 165/106 mary mechanism responsible for this heattransfer.

a a ,,d O o o 0 a 0 1337/94 0; O a a a a o /3 g- -20 1i alsaslsssPATENTED JAN! 1 1972 SHEET 1 OF 3 5 f m m m fin r .w i Ma 42 PM HEATEXCHANGER USING THERMAL CONVECTION TUBES CONTRACTUAL ORIGIN OF THEINVENTION BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustratedin the drawings, of which: FIG. 1 is a cross-sectional view of the heatexchanger eleor under, a contract with the UNITED STATES ATOMIC ENERGYCOMMISSION.

BACKGROUND OF THE INVENTION To date, the development of heat exchangersfor use in liquid metal breeder reactors has been predominantly in thearea of conventional type shell and tube heat exchangers. In order toisolate the water from the primary sodium which circulates through thereactor and which is radioactive, a secondary sodium loop is employed.This requires two heat exchangers, one to transfer heat from the primarysodium to the secondary sodium and one to transfer heat from thesecondary sodium to the water. However, the water is still adjacentliquid sodium and hazards of a sodium-water reaction resulting from aleak are present. Another problem is in the welds used in theconstruction of the shell and tube heat exchangers. The welded regionsare subject to considerable stress and are the source of many failures.

Heat pipes are known to have the ability to transfer relatively largequantities of heat at small thermal gradients. However, heat pipes havehad relatively short lifetimes compared to the requirements for nuclearreactor application. The interaction of the working fluid with the heatpipe walls creates a residue of material which has been found toaccumulate at the boiler section of the heat pipe. The residue acts toobstruct the return flow of the working fluid through the wick passagesof the heat pipe.

It is therefore an object of this invention to provide a heat exchangerwherein sodium-water contact does not occur subsequent to a single tuberupture.

Another object of this invention is to provide a heat exchanger whereinexchanger elements are not rigidly fastened at each end, thussubstantially reducing stress problems.

Another object of this invention is to provide a single heat exchangerin place of the primary and intermediate heat exchangers used inliquid-metal nuclear reactors.

Another object of this invention is to provide a heat exchanger whereinboiling and superheating take place in separate chambers.

Another object of this invention is to provide a heat exchanger having alifetime compatible with nuclear reactor requirements by employingwickless heat pipes, thermal convection tubes."

SUMMARY OF THE INVENTION In practicing this invention a heat exchangeris provided having two regions with one region positioned above theother and separated therefrom by a separation wall. The lower regioncontains a first fluid which may be, for example, liquid sodium. Thesecond region contains a second cooler fluid which may be, for example,water. A sealed hollow tubular member extends from the lower regionthrough the separation wall to the upper region, with its longitudinalaxis substantially vertical. The lower portion of the hollow tube whichis within the lower region is substantially full of a third fluid in itsliquid state. The third fluid may be, for example, mercury. Inoperation, heat from the sodium would be transferred to the mercurywhich boils. The mercury vapor rises to the upper portion of the tubewhich is within the upper chamber. Heat is transferred from the mercuryvapor to the water, heating and boiling the water while simultaneouslycondensing the mercury. The condensed mercury falls by gravity to thelower portion of the hollow tube where it is again heated by the sodium.Vapor from the boiling water may be transferred to a second chamber andsuperheated in a similar manner.

FIG. 2 is a cross-sectional view of a heat exchanger having separateboiling and superheating chambers.

FIGS. 3 and 4 are cross-sectional views of another embodiment of heatexchangers having separate boiling and superheating chambers.

FIG. 5 is a schematic showing the heat transfer path of prior art shelland tube heat exchangers.

FIG. 6 is a schematic showing the heat transfer path in the heatexchanger of this invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a schematicrepresentation of a cross section of the cylindrical thermal convectiontube of this invention. Heat from a primary fluid 14, which may be, forexample, sodium, is transferred to the working fluid 19 inside of thetube 10, causing the working fluid 19 to boil. The vapor 19A rises intothe condenser section 11 of tube 10 where heat is removed to a secondaryfluid 16, for example, water, thus condensing the working fluid 19.Gravitational forces cause the condensed working fluid 198 to return tothe heater section 13 of tube 10. A downcomer 20 is used to promote thenatural circulation of the working fluid and thus increase theefficiency of the system. A wall 17 separates the primary fluid 14 fromthe secondary fluid 16. Wall 17 can be made as thick as is required bystrength and shielding considerations without significantly affectingthe heat transfer through the thermal convection tube 10.

The axial temperature of the working fluid 19 in the thermal convectiontube 10 varies only a few degrees from the saturation temperature at theliquid-vapor interface. This tube, like a heat pipe, represents a methodof sustaining large heat fluxes over significant distances at nearlyisothermal conditions. However, in the case of the thermal convectiontube, the absence of a wick material permits the heat flux per unit areato be greatly increased over the normal heat pipe by allowing forboiling of the working fluid without decreasing tube lifetime.

FIG. 2 illustrates a thermal convection tube steam generator which may,for example, use sodium as the primary fluid and water as the secondaryfluid. However, the invention is not restricted to use of these twofluids. The primary fluid enters through inlet 21, circulates past tube38 in chamber 22, tube 34 in chamber 23 and tube 31 in chamber 24. Thefluid exits through outlet 25. As the primary fluid moves past each ofthe tubes, heat is transferred to the working fluid within the tube. Inthe upper portion of the steam generator, a secondary fluid such aswater in the liquid state enters through inlet 28 into section 29. Heatfrom the working fluid within tube 31 is transferred to the water insection 29, causing the water to boil. The vapors are carried intosection 33 where heat from tube 34 acts to superheat the vapor. Thevapor is then transferred to section 37 where additional superheatingtakes place by the transfer of heat from thermal tube 38. Thesuperheated vapor exits through outlet 39. Separation of the stagesallows the water to be boiled most efficiently in the nucleate boilingregion in the boiler stage and then superheated in separate stages.

FIGS. 3 and 4 show a steam generator in which the tubes and stages areconcentrically arranged. The primary fluid enters through inlets 42 and43 in the lower portion of the generator and flows past thermal tubes 59and 55 in sections 45 and 46. The primary fluid then flows into thecenter chamber 47, circulates past tubes 51 and is removed throughoutlet 49. Water enters through an inlet (not shown) into the upperchamber 52, where heat from tubes 51 causes the water to boil,generating steam. The steam flows radially to the inlet to chamber 53where it is heated by heat transferred from tubes 55. The superheatedsteam flows radially into chamber 58, where it is superheated to highertemperatures by heat transferred from tubes 59. The superheated steamexhausts through outlets 61 and 62.

Referring to FIG. 5, there is shown the heat path from the primarysodium 65 to water 66 in the prior art single-pass shell and tube steamgeneration system. The path is through the secondary sodium 68 andthrough tube walls 69 and 71.

Referring to FIG. 6, there are shown two paths of heat transfer fromprimary sodium 73 to the water 74 in the thermal convection system. Onepath is through the working fluid 76, which may be, for example,mercury, plus two tube walls 78 and 79. This heat path is the same asthe heat path in the shell and tube system with the secondary sodiumreplaced by the working fluid 76. The second path is from the primarysodium 73 through the separation wall 80 (17 in FIG. 1). The thicknessof the separation wall 80 may be very large without significantlyreducing the performance of the thermal convection tube; most of theheat transfer occurs through the thermal convection tube. The workingfluid 76 takes the place of the secondary sodium of the conventionalsystem and thus an intermediate heat exchanger is not required. Further,the water is not immediately adjacent the sodium, so that the problemsassociated with sodium-water leaks are not present.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A heat exchanger system for use in generating steam with heat from asodium fluid which has been heated in a nuclear reactor, including incombination, a first chamber coupled to the reactor for receiving saidsodium fluid therefrom, said first chamber having a top wall, a secondchamber positioned above said first chamber with said top wall of saidfirst chamber forming the bottom wall of said second chamber, said topand bottom wall forming a separation wall between said top and bottomchambers, said second chamber further containing water, a plurality ofscaled hollow members each having an upper portion and a lower portion,each of said sealed hollow members being placed in a substantiallyvertical position and extending through said separation wall with saidlower portion in said first chamber and said upper portion in saidsecond chamber, said sealed hollow members being mechanically secured tosaid separation wall at the middle portion of said sealed hollowmembers, each of said sealed hollow members being in the form of a firsttube having sides and closed bottom and top ends, with said lowerportion thereof being substantially filled with liquid mercury, a secondtube having open top and bottom ends and centrally located withreference to said sides of said first tube, said bottom end of saidsecond tube being spaced away from said bottom end of said first tube,said top end of said second tube being positioned below the surface ofsaid liquid mercury in said lower portion of said sealed hollow member,said heated sodium acting to heat said liquid mercury to form a mercurygas, said mercury gas rising to said upper portion and transferring heatto said water while in said upper portion to generate steam in saidsecond chamber, said gas condensing to liquid mercury after said heattransfer and returning to said lower portion, said second tube acting asa downcomer to promote circulation of said liquid mercury in said lowerportion.

1. A heat exchanger system for use in generating steam with heat from asodium fluid which has been heated in a nuclear reactor, including incombination, a first chamber coupled to the reactor for receiving saidsodium fluid therefrom, said first chamber having a top wall, a secondchamber positioned above said first chamber with said top wall of saidfirst chamber forming the bottom wall of said second chamber, said topand bottom wall forming a separation wall between said top and bottomchambers, said second chamber further containing water, a plurality ofsealed hollow members each having an upper portion and a lower portion,each of said sealed hollow members being placed in a substantiallyvertical position and extending through said separation wall with saidlower portion in said first chamber and said upper portion in saidsecond chamber, said sealed hollow members being mechanically secured tosaid separation wall at the middle portion of said sealed hollowmembers, each of said sealed hollow members being in the form of a firsttube having sides and closed bottom and top ends, with said lowerportion thereof being substantially filled with liquid mercury, a secondtube having open top and bottom ends and centrally located withreference to said sides of said first tube, said bottom end of saidsecond tube being spaced away from said bottom end of said first tube,said top end of said second tube being positioned below the surface ofsaid liquid mercury in said lower portion of said sealed hollow member,said heated sodium acting to heat said liquid mercury to form a mercurygas, said mercury gas rising to said upper portion and transferring heaTto said water while in said upper portion to generate steam in saidsecond chamber, said gas condensing to liquid mercury after said heattransfer and returning to said lower portion, said second tube acting asa downcomer to promote circulation of said liquid mercury in said lowerportion.